Beacon tracking receiver

ABSTRACT

A radio frequency receiver is responsive to signals radiated by a remote transmitter. The receiver includes at least one pair of antennas separated by approximately one-half wavelength at the transmitter frequency. The phases of the signals detected by the antennas are compared for the purpose of determining the relative direction to the transmitter.

This invention relates to radio frequency receivers and in particular toa phase comparison radio frequency receiver.

There are many applications for a radio frequency receiver whichprovides an indication of the direction to a source of electromagneticenergy. While the invention will be illustrated by a system for theapprehension of the perpetrators of a robbery, other applications of theinvention will immediately suggest themselves. Such other applications,for example, include the location of victims of abduction, location andrescue of downed aircraft, and tracking of motor vehicles.

Briefly, in the preferred embodiment the receiver employs three antennaswhich are located at the vertices of a right triangle. The sides of thetriangle have lengths of approximately one-half wavelength at thefrequency to be detected. One side of the triangle may be aligned withthe fore/aft axis of a vehicle carrying the receiver while the otherside of the triangle is perpendicular to the fore/aft axis. The receiverincludes two intermediate frequency channels, one of which is coupled tothe antenna located at the ninety degree angle of the triangle. Theother intermediate frequency channel may be selectably connected toeither of the remaining antennas. In the preferred embodiment each ofthe intermediate frequency channels is a triple down-conversion channelwhich serves to reduce the frequency of the signal for detectionpurposes. The local oscillator signals employed for down-conversion arecommon between the two channels, thereby insuring that the phases of thesignals in the two intermediate frequency channels will be preservedduring the frequency translation process. The phase difference betweenthe two signals is determined by a phase detector responsive to theoutputs of the two intermediate frequency channels. When the twochannels are coupled to the antennas which are located along thefore/aft axis this phase difference is indicative of whether thetransmitter is located ahead of or behind the receiver. Similarly, whenthe laterally aligned antennas are used this phase difference isindicative of whether the transmitter is located to the right of or tothe left of the receiver. In the preferred embodiment an AGC signalgenerated in one of the intermediate frequency channels is used as ameans for indicating the relative proximity of the transmitter.

It is therefore an object of the invention to provide a radio frequencyreceiver for determining the direction to a source of electromagneticenergy.

It is another object of the invention to provide a directional radiofrequency receiver employing phase comparison techniques.

It is a further object of the invention to provide a radio frequencyreceiver for generating electrical signals representative of thedirection to and the distance to a source of electromagnetic energy.

Other objects and features of the invention will be made obvious by aconsideration of the following detailed description in connection withthe drawings wherein:

FIG. 1 is a diagram showing the elements of the beacon tracking system.

FIG. 2 shows a perspective view of the display unit.

FIGS. 3a and 3b illustrate the internal configuration of a boguscurrency packet.

FIG. 4 shows one switch configuration used to energize the transmitter.

FIG. 5 is a block diagram of the transmitter.

FIG. 6 is a schematic diagram of the transmitter.

FIG. 7 is a block diagram of the beacon tracking receiver and displaysystem.

FIG. 8 is a schematic diagram of the preamplifier used in the receiver.

FIG. 9 is a schematic diagram of the first local oscillator.

FIG. 10 is a schematic diagram of the first mixer and first IFamplifier.

FIG. 11 is a schematic diagram showing the second local oscillator,mixer and IF amplifier.

FIG. 12 is a schematic diagram showing the third local oscillator mixerand IF amplifier, and the phase detector.

FIG. 13 shows waveforms illustrating the phase detector operation.

FIG. 14 is a block diagram of the display circuits.

With reference to FIG. 1 there are shown, in symbolic form, the elementsof the beacon tracking system as they appear in the preferredembodiment. A bogus currency packet 10 which has been removed from itsproper repository contains a radio frequency transmitter which, as aresult of the unauthorized removal, is in a transmitting mode. A groundplane 12, which may comprise the roof of a police cruiser, supportsthree RF receiving antennas 14, 16 and 18. Antennas 14, 16 and 18 areselected to be efficient receivers at the frequency transmitted by boguscurrency packet 10. Antennas 14 and 16 are located along a line which isperpendicular to the direction of travel of the police cruiser whileantennas 14 and 18 are located along a line which is parallel to thedirection of travel. In the preferred embodiment the separation betweenantennas 14 and 16 and between antennas 14 and 18 is approximately onehalf wavelength at the frequency transmitted by bogus currency packet10. A beacon tracking receiver 20 and its associated display unit 22 maygenerally be located inside the police cruiser. As indicatedsymbolically by line 24, the electrical signals received by antennas 14,16 and 18 are coupled to beacon tracking receiver 20 while receiver 20is electrically connected to display unit 22 as indicated by line 26.

Operationally it will be seen that when the transmitter is directly infront of or behind the police cruiser, the electrical signals receivedby antennas 14, and 16 will be in phase. If, however, the cruiser isoriented such that bogus currency packet 10 is located to the left ofthe cruiser's fore/aft axis, then the phase of the signal received byantenna 16 will lag that of the signal received by antenna 14. In asimilar manner the signals received by antennas 14 and 18 will be inphase when the transmitter is directly abreast of the cruiser but willbe out of phase at those times when the transmitter is either ahead ofor behind the lateral axis of the cruiser. The signals received by thethree antennas are processed in beacon tracking receiver 20 so as toprovide, through display unit 22, a visual and audible indication of therelative direction of the bogus currency packet 10.

The external configuration of display unit 22 is illustrated in FIG. 2.The unit is located in a case which is adapted to be convenientlymounted on the dash of the police cruiser. A front panel meter 28 isresponsive to the signals received by antennas 14 and 16 afterprocessing in beacon tracking receiver 20 to provide a visual indicationas to whether the bogus currency packet 10 is left or right of thefront/rear axis of the cruiser. Alternatively, when front/rear switch 30is depressed, front panel meter 28 is responsive to the signals receivedby antennas 14 and 18, after processing by beacon tracking receiver 20,to show the location of bogus currency packet 10 relative to the lateralaxis of the cruiser. Also, when front/rear switch 30 is depressed,either front or rear light 32 or 34 will be illuminated to provide analternative display of the location of bogus currency packet 10 relativeto the cruiser lateral axis. The amplitude of the signals received bythe antennas is employed by beacon tracking receiver 20 to provide avisual indication of the range to the bogus currency packet 10 on frontpanel distance meter 36.

While not shown expressly in FIG. 2, display unit 22 also providesaudible indications to the vehicle operator. When the antennas receive asignal with an amplitude exceeding a preselected threshold, display unit22 emits an audible tone signal which alerts the operator to the factthat a bogus currency packet is transmitting. The frequency of theaudible tone signal is related to the strength of the signals receivedby the antennas so as to provide to the operator an audible indicationof the relative range to the transmitting bogus currency packet 10. Thisaudible tone is also amplitude modulated when the angle between thevector to the transmitter and the direction of travel of the policecruiser exceeds a preselected angle. In this way the operator isprovided with an audible indication that a turn should be made. A volumecontrol knob 38 on the front panel of the display unit 22 permitsadjusting the volume of the audible tone signal for the convenience ofthe operator but cannot be completely turned off.

In FIGS. 3A and 3B there are illustrated perspective views of portionsof a currency packet and of the transmitter which is secreted in theinterior portion of the currency packet. In the preferred embodiment aportion of the bills from a conventional currency packet comprise thebase member 50 of the structure on which is located a thin rectangularmember 52 made of a polymide film of the type sold under the trademarkKAPTON by E. I. duPont de Nemours Co. Member 52 has peripheraldimensions slightly smaller than those of a bill. Formed on member 52 isa copper etch segment 54 which comprises a loop antenna. Contact betweenantenna 54 and printed circuit board 56 is made in the general area ofpoints 58 and 60. Printed circuit board 56 supports the electricalcomponents which make up the transmitter circuit. Battery 62 providesthe electrical energy for the transmitter. Battery 62 may be an 8.4 volt(six cells) battery available from Mallory Battery Co.

The remainder of the conventional currency packet 64 is shown invertedin FIG. 3A so as to expose recess 66 which is formed by providing arectangular opening in a portion of the bills comprising the remainderof the packet. Recess 66 is provided for the purpose of accomodating thecomponents on printed circuit board 56 as well as battery 62, therebypermitting the entire transmitter structure to be hidden internal to thecurrency packet.

The transmitter, while located in the cash drawer, is decoupled from thebattery and remains in a quiescent state. The act of removing thecurrency pack from the cash drawer activates the transmitter causing itto begin broadcasting at the predetermined frequency. One means ofactivation is illustrated in FIG. 4 where is shown a portion 70 of thesurface of printed circuit board 56. There is located on the printedcircuit board a switch 72 comprised of electrically conductive segments74 and 76. Segment 74 may be formed as a part of the overallmetallization pattern of the printed circuit board. Segment 76 has abase portion which is affixed to the substrate material of the printedcircuit board and a cantilevered portion which extends over and makeselectrical contact with segment 74. The cantilevered portion of segment76 constitutes a spring exerting firm mechanical pressure on underlyingsegment 74 so as to insure electrical contact at all times. The switchcomprised of segments 74 and 76 is placed in the line coupling battery62 to the transmitter. When the currency pack is in a cash drawer orother proper receptacle, switch 72 is held in an open position by meansof insulating strip 78 which extends into recess 66 of the currencypacket as shown in FIG. 3A. Insulating strip 78, which may be of aconvenient material such as MYLAR is located so that the portion of thestrip extending into recess 66 is located between conductive segment 74and the cantilevered portion of conductive segment 76 therebyinterrupting the continuous electrical path through switch 72. Theportion of insulating strip 78 extending beyond the periphery of thecurrency pack is mechanically attached to the cash drawer by means oftape or other suitable means for affixation. In this way, conductivestrip 78 prevents the flow of current from battery 62 to the transmitterwhen the currency pack is in the cash drawer. When the currency pack isremoved from the cash drawer, insulating strip 78 is withdrawn from thecurrency pack thereby permitting the contacts of switch 72 to close andelectrically connect battery 62 to the transmitter.

FIG. 5 is a block diagram of the transmitter. Oscillator 80 is a crystalcontrolled oscillator operating at 115.667 mhz. The frequency of theoutput signal from oscillator 80 is tripled in frequency multiplier 82so as to provide a 347 mhz CW signal. Modulation signal generator 84provides a 570 hertz signal which is used to amplitude modulate the 347mhz CW signal. The modulated CW signal is amplified by amplifier 86 andthence coupled to antenna 88 which is in the form of loop antenna 54 asseen in FIG. 3B.

A schematic diagram of the transmitter is shown in FIG. 6. Thecomponents included within dashed rectangle 90 comprise the crystalcontrolled oscillator 80 of FIG. 5. This will be recognized as aColpitts oscillator, modified by the inclusion of a crystal, Y1, betweenthe emitter of transistor Q1 and the junction between capacitors C3 andC4. This crystal oscillator operates in a series resonant mode at afrequency of 115.667 mhz and establishes a frequency stability of ±578Hz (±5 ppm).

The components located within dashed rectangle 92 comprise frequencymultiplier 82 of FIG. 5. The output signal from oscillator 80 is coupledto frequency multiplier 82 by a matching circuit comprising capacitorsC7, C8, C9 and inductor L2. The matching circuit is tuned by properlyadjusting variable capacitor C8. Frequency multiplier 82 is an amplifierwhich is driven by the output of oscillator 80 so as to operate in asubstantially non-linear mode and thereby generate a substantialcomponent at the third harmonic of the frequency produced by oscillator80. The fundamental component appearing at the collector of transistorQ2, which is the largest single component other than the third harmoniccomponent, is shunted to ground by the combination of capacitor C11 andinductor L3, this combination being series resonant at the fundamentalfrequency. Further filtering of the multiplied signal is provided by thefilter comprised of capacitors C12-C15 and inductor L4. Transistor Q5and its associated components comprise amplifier 86 of FIG. 5. Thisamplifier network increases the power level of the third harmoniccomponent to a level suitable for transmission by the antenna(100-150mw).

In the schematic diagram of FIG. 6 switch S1 is normally open but isclosed upon withdrawal of the currency packet from its designatedreceptacle thereby providing a positive voltage supply to thetransmitter. Switch S1 may, for example, be switch 72 as shown in FIG.4. The 8.4 volt battery voltage is reduced to a regulated voltage by thevoltage regulator comprising transistor Q7, resistor R14 and regulatordiode VR1. In the preferred embodiment regulator diode VR1 is a 5.1 voltzener diode which serves to set the regulated voltage appearing at theemitter of transistor Q7 at approximately 4.5 volts.

The components located within dashed rectangle 94 comprise modulationsignal generator 84 of FIG. 5. This portion of the transmitter circuitis an astable multivibrator as is well known in the art. Fine tuning ofthe multivibrator to its frequency of 570 hertz is accomplished byselecting capacitors C20 and C21. The square wave output of the astablemultivibrator drives a current switch composed of transistors Q6 and Q8.When the output of the multivibrator is in the high state, bothtransistors Q6 and Q8 are turned on hard so as to provide substantiallythe battery voltage to that portion of the transmitter network includingfrequency multiplier 82 and amplifier 86. Alternatively, when themultivibrator output is in the low state transistors Q6 and Q8 areeffectively turned off and the battery supply voltage is disconnectedfrom the frequency multiplier and amplifier. Consequently, frequencymultiplier 82 can operate to triple the oscillator output only duringalternative half cycles of the waveform provided by modulation signalgenerator 84. This amounts to square wave amplitude modulation of thethird harmonic frequency at the 570 hertz rate produced by themultivibrator. This amplitude modulation is useful in the yet to bedescribed receiver circuit for the purpose of identifying the signaltransmitted by the transmitter.

FIG. 7 is a block diagram of the receiver and display system. In a firstmode of operation the receiver monitors the signals appearing on rightantenna 16 and left antenna 14 and utilizes the relative phases of thesetwo signals to determine the direction to a transmitter relative to areceiver front/rear axis. The receiver comprises two identical tripleconversion channels, one for each of the antenna signals. The tripleconversion process is utilized to reduce the received frequency to aconvenient low frequency for the measurement of the phase differencebetween the signals. The triple conversion channel for the signaldetected by antenna 14 comprises those elements located within dashedrectangle 104. The triple conversion channel for the signal detected byantenna 16 is represented by dashed rectangle 106 and includes elementsidentical to those shown expressly within rectangle 104. The localoscillator signals necessary to enable the triple conversion mixing areprovided by first local oscillator 108 second local oscillator 110 andthird local oscillator 112. Each of these local oscillators provides acommon signal to both of the triple conversion channels. In this way itis insured that the relative phases of the two signals are preservedduring the triple conversion process. The frequencies provided byoscillators 108, 110 and 112 are respectively, 322.5 mhz, 24.045 mhz and479 khz. With a transmitter frequency of 347mhz, this results in IFfrequencies of 24.5 mhz, 455 khz and 24 khz.

Considering in detail the elements comprising one of the tripleconversion channels, the signal detected by antenna 14 is coupled byline 114 to preamplifier 116. The output of preamplifier 116 is combinedin mixer 118 with the output of first local oscillator 108 to provide anintermediate frequency to first IF amplifier 120. The output ofamplifier 120 is mixed in mixer 122 with the signal provided by secondlocal oscillator 110 and coupled then to second IF amplifier 124. Oneoutput of second IF amplifier 124, appearing on line 126, is an AGCsignal which, inter alia, is coupled by line 128 back to first IFamplifier 120. The use of AGC in amplifiers 120 and 124 permits thesystem to respond to a wide dynamic range of input signals. A secondoutput from second IF amplifier 124 appearing on line 130 is combined inmixer 132 with the signal provided by third local oscillator 112 toprovide an intermediate frequency signal for third IF amplifier 134. Theoutput of amplifier 134 provides one of the inputs to phase detector136.

The signal detected by antenna 16 is coupled by coaxial relay 138 to thesecond triple conversion channel represented by rectangle 106. Coaxialrelay 138 may be a Model RFB-4502 unit manufactured by Hi-G Inc. ofWindsor Locks, Ct. The output of the second triple conversion channelappearing on line 140 provides a second input to phase detector 136.Phase detector 136 compares the phases of the two signals and providesan output to direction meter 142 which is front panel meter 28 of FIG.2.

The AGC signal provided by amplifier 124 is also coupled by line 144 todistance indicator 146 which represents front panel meter 36 of FIG. 2.When the transmitter whose signals are being detected by antenna 14 isin relatively close proximity to the receiver, the received signals willbe relatively large thereby resulting in a strong AGC signal at theoutput of amplifier 124. This strong signal appearing on line 144 causesdistance indicator 146 to read upscale and thereby indicate the nearproximity of the transmitter. Conversely, a distant transmitter wouldresult in a weak AGC signal on line 144 and consequently a down scalereading on distance indicator 146.

The output signal from amplifier 124 appearing on line 130 is coupled toalarm detector 148. Alarm detector 148 detects the presence or absenceof a 570 hertz modulation on the output of amplifier 124 and, in thepresence of such modulation, enables tone generator 150. Tone generator150 in turn provides an audible frequency tone to speaker 152. It willbe seen from the foregoing that speaker 152 will provide an audible toneonly when alarm detector 148 senses the 570 hertz modulation which isemployed in the transmitters forming a portion of the beacon trackingsystem. It will be noted that tone generator 150 is also responsive tothe AGC output of amplifier 124, this output appearing on line 154. Tonegenerator 150 includes a voltage controlled oscillator having a variablefrequency controlled by the level of the AGC signal appearing on line154. As the level of this AGC signal increases, indicating a lesseningdistance between the receiver and the transmitter, the frequencyprovided by tone generator 150 increases. In this way, an operator isprovided with an audible indication of the relative proximity of thetransmitter as well as the visual indication provided by distanceindicator 146.

It will further be noted that the output of phase detector 136 iscoupled by line 156 to large angle detector 158. Large angle detector158 provides a gating signal on line 160 at those times when the vectorto the transmitter deviates from the front/rear axis of the receiver bymore than a selectable threshold angle. This gating signal in turn gatesthe output of tone generator 150 alternatively on and off in a periodicfashion. The period of one on/off cycle is approximately 1/4 to 1/2second. As a result, the audible tone provided by speaker 152 isintermittent (beeps), thereby giving an operator an audible indicationthat a turn is required.

When front/rear switch 30 of FIG. 2 is depressed, coaxial relay 138decouples right antenna 16 from the input of triple conversion channel106 and couples rear antenna 18 to the input of channel 106. Whenoperating in this mode, direction meter 142, responding to the output ofphase detector 136, indicates to an operator the direction to atransmitter relative to a lateral axis of the receiver. At the same timefront/rear lamp assembly 164 is activated to provide the operator with asecond readily identifiable visual indication of the relative locationof the transmitter. This feature resolves the 180° ambiguity presentwhen only the left and right antennas are used.

With regard to the individual elements of the receiver, antennas 14, 16and 18 may be 5/8 λ whip antennas. The spring base of each antenna issufficiently stiff to prevent substantial deflection by the air streampassing the vehicle, but flexible enough to avert damage in car washes.The upper extremity of the antenna is extendible for fine tuning. In apolice cruiser application the antennas may be mounted on the roof ofthe cruiser with the spacing between the right and left antenna andbetween the front and rear antenna approximately 1/2 wavelength or lessat the carrier frequency of the transmitter.

A schematic diagram of preamplifier 116 appears in FIG. 8. Thepreamplifier is a conventional two stage FET amplifier. The circuitprovides approximately 26 dB of gain with a noise figure ofapproximately 1.5 dB and a bandwidth of approximately 4 mhz.

First local oscillator 108 is illustrated in schematic form in FIG. 9.The input to resistor R18 labeled "input from stable oscillator" is a107.5 mhz signal provided by an oven-stabilized crystal oscillator. Inthe preferred embodiment this oscillator has a frequency stability ofbetter than 1/2 part per million over a temperature range of 0° to +50°C. A suitable oscillator is available from Ovenaire Incorporated ofCharlottesville, Va. In FIG. 9 the stage including transistor Q11 is abuffer stage to provide isolation and impedance matching between theoutput of the oven-stabilized oscillator and the remainder of the firstlocal oscillator circuit. The output signal from the buffer stage is ofsufficient amplitude to drive the frequency multiplier stage includingtransistor Q12 so that a third harmonic component appears at thecollector of transistor Q12. This third harmonic component is at thedesired first local oscillator frequency, that is 322.5 mhz. The thirdharmonic component is further amplified in output stages includingtransistors Q13 and Q14. The signals produced by these two output stagesare the first local oscillator output signals utilized in the tripleconversion channels of the receiver. The signal coupled out of inductorL17, for example, is coupled to mixer 118 of FIG. 7.

Mixer 118 and first IF amplifier 120 are illustrated with greater detailin FIG. 10. For the purposes of clarity in this description, when thesame component appears in more than one figure it is assigned the samereference designator in each of the figures. In the preferred embodimentmixer 118 is a double-balanced mixer Model MD108 manufactured by AnzacElectronics of Waltham, Mass. In FIG. 10 the numbers within the boxdesignating mixer 118 indicate the proper external connections for theeight terminals of the MD108 mixer.

The 24.5 mhz output signal from mixer 118 is bandpass filtered incrystal filter 182. This component is readily available commercially andrequires no further discussion here. The filtered mixer output isamplified in amplifier 184. A suitable amplifier for this purpose is aModel MC1590G high frequency amplifier manufactured by Motorola Inc. ofPheonix, Ariz. The terminal labeled "AGC (+) signal" is connected toline 128 of FIG. 7 to receive the AGC signal generated in second IFamplifier 124.

Second local oscillator 110, mixer 122, and second IF amplifier 124 areshown in schematic form in FIG. 11. The second local oscillator iscomprised of transistor Q16 and its associated components. Crystal Y2functions as a series resonant circuit at its resonant frequency of24.045 mhz. At this frequency, therefore, the emitter of transistor Q16has a low impedance feedback path to the junction point betweencapacitors C74 and C75 and the circuit functions essentially as aclassical colpitts oscillator. In FIG. 11 the second local oscillatorprovides a signal to the second IF amplifier for one channel of thetriple conversion receiver. While not shown expressly, it will berecalled that the oscillator also provides a signal to the second IFamplifier in the second channel of the triple conversion receiver.Second IF amplifier 124 includes two stages of amplification, thesebeing provided by integrated circuit IF amplifiers 190 and 194. Bothintegrated circuit IF amplifiers 190 and 194 may be a Model MC1350 IFamplifier manufactured by Motorola Inc. of Phoenix, Ariz. Thedouble-ended output of amplifier 190 has a center frequency of 455 khzand is coupled by transformer T2 to IF bandpass filter 192. Bandpassfilter 192 may be any of a wide variety of commercially available IFfilters. The output of filter 192 is further amplified by IF amplifier194 whose double-ended output is coupled by transformer T3 to the outputterminals of the second IF amplifier. To avoid unnecessary complexity inthe schematic diagram of FIG. 11, those points of the circuit which areelectrically common with junction point 196 have all been indicated by asmall rectangle enclosing a diagonal cross.

The 24.045 mhz second local oscillator signal is resistively coupledinto the AGC terminal of amplifier 190, that is terminal 5. Amplifier190, as well as forming a portion of second IF amplifier 124, alsoperforms the function of mixer 122.

The AGC signal appearing on line 126 of FIG. 7 is generated in second IFamplifier 124. This AGC signal is dependent on the amplitude of thesignal appearing at the output of amplifier 190, that is, terminal 8 andon the amplitude of the signal appearing at the output of amplifier 194,that is, terminal 1. Because of the narrow passband of filter 192,output noise from amplifier 190 may be of sufficient amplitude tosaturate this amplifier while the filtered signal passing through 194will not saturate the latter amplifier. In order to counter thiscondition the output of amplifier 190 generates an AGC signal to reducethe gain of the various amplifiers.

Considering the development of an AGC voltage at the output of amplifier190, it will be noted that the combination of resistors R59 and R60along with zener diode VR2 establishes a reference DC voltage at the nongrounded side of capacitor C64. Capacitor C63 is a small couplingcapacitor which serves to couple the signal from the output of amplifier190 to the AGC circuit. Capacitor C64 is an RF bypass capacitor whichserves to shunt signal currents to ground and thereby maintain a stableDC voltage at its ungrounded terminal. As a result, when the signal atterminal 8 of amplifier 190 swings negative the cathode of rectifier CR5is clamped to a voltage approximately 0.6 volts below the stable DCreference voltage, 0.6 volts being the forward bias diode drop ofrectifier CR5. It will be recognized by those skilled in the art,therefore, that the AGC detector functions as a peak-to-peak detector.

The AGC detector which is coupled to the output of amplifier 194operates in an identical fashion so that large signals appearing at theoutput of either of amplifiers 190 or 194 will result in an AGC voltageappearing at the base of transistor Q15 and charging of capacitor C71.Transistor Q15 is an emitter follower which provides impedance matchinginto the lowpass filter comprised of resistor R51 and capacitor C72 andthen to the AGC (+) signal terminal of the second IF amplifier. Thesignal appearing at this last mentioned terminal is coupled back to theAGC input of first IF amplifier 120. It will be noted that this samesignal is coupled by resistor R40 back to the AGC input of amplifier 190and by resistor R49 back to the AGC input amplifier 194. As seen in FIG.7 this AGC signal is also coupled by line 144 to distance indicator 146and by line 154 to tone generator 150.

Distance indicator 146 is a one milliampere full scale ammeter. Thisammeter is not connected directly between the AGC (+) signal terminaland ground since the voltage at the AGC (+) signal terminal is non-zeroeven with no signal applied to the second IF amplifier. This zero signalAGC level is approximately three diode drops below the reference voltageestablished by the combination of resistors R59 and R60 and zener diodeVR2. If the ammeter were so connected it would indicate a non-zeroreading even under the zero signal condition. Proper operation isprovided by the circuit of FIG. 11 where an AGC (-) signal is generatedby the combination of variable resistances R61 and R62. The distanceindicator ammeter is coupled between these two AGC signal terminals andvariable resistances R61 and R62 are adjusted to provide a zero ammeterreading under zero signal conditions.

The face of the ammeter is modified so that the zero signal end of themeter scale is indicative of a distant transmitter while up-scalereadings, reflective of an increasing level of AGC signal, areindicative of nearer transmitter locations. The face of distance meter146, in addition to having numbers indicative of the approximatedistance to the transmitter, is color coded in distance ranges to give areadily interpretable indication of distance to an operator.

In FIG. 12 there is shown a schematic diagram which includes third localoscillator 112, mixer 132, third IF amplifier 134 and phase detector136. In FIG. 12 those components lying above dashed line 200 comprisemixer 132 and third IF amplifier 134 for triple conversion channel A ofthe receiver. Rectangle 206 represents the corresponding components fortriple conversion channel B of the receiver. Transistor Q20 and itsassociated components comprise the third local oscillator which operatesat a frequency of 479 khz under the control of crystal Y3. Again, thiscircuit will be recognized as a crystal colpitts oscillator. Theoscillator output is coupled to the AGC input of IF amplifier 202 whichfunctions both as mixer 132 and as a portion of third IF amplifier 134.IF amplifier 202 may be similar to a Model SN76660N integrated circuitmanufactured by Texas Instruments Incorporated of Dallas, Texas.Alternatively, it may be similar to a Model TAA661B integrated circuitmanufactured by SGS-ATES Semiconductor Corporation of Newtonville, Mass.The output of amplifier 202 is coupled to a phase splitter whichincludes transistor Q17. The two outputs from the phase splitter arefurther phase shifted by the network comprising resistors R72 and R74and capacitors C94 and C95. The two phase shifted signals areselectively combined by variable resistance R75 so as to provide, at thebase of transistor Q18, a signal whose phase is variable with respect tothe phase of the signal appearing at the base of transistor Q17.Transistor Q18 provides additional gain to this signal which is thenhard limited by amplifier 204. Amplifier 204 may be similar to an RCAModel CA3076 integrated circuit. The hard limited signal is buffered bytransistor Q19 and then coupled to NAND circuit U1 which comprises afirst stage of phase detector 136.

Assuming that the receiver is receiving a signal at the transmitterfrequency of 347 mhz, then because of the hard limiting in amplifier 204the waveform appearing at the collector of transistor Q19 is a 24 khzsquare wave. This square wave is represented by waveform 210 in thewaveform diagram of FIG. 13. It provides one input to NAND gate U1, theother input of which is coupled to a positive voltage supply. NAND gatesU1 through U8 may each be a portion of a model SN 7400N quadruple NANDgate manufactured by Texas Instruments Incorporated of Dallas, Texas.The output of NAND gate U1 in response to waveform 210 is waveform 211.This signal provides one input to NAND gate U3. It is also low passfiltered by the combination of resistor R88 and capacitor C110 toprovide one input to NAND gate U2, the other input of which is alsoreferenced to the positive voltage supply. The low pass filteredwaveform appearing at the junction between resistor R88 and capacitorC110 is represented by waveform 212. This waveform is an approximatereplica of waveform 211 but is slightly delayed at its leading andtrailing edges by the charging time of capacitor C110. As a result, theoutput of NAND gate U2, represented by waveform 213, is a square wavewhose transition times lag slightly the transition times of waveform211. Waveforms 211 and 213 are the inputs to NAND gate U3, the output ofwhich is high at all times except for a brief interval following eachnegative going excursion of waveform 211, the output of NAND gate U3being represented by waveform 214.

The waveforms of FIG. 13 have been drawn for the situation when thesignals received by the two active antennas are in phase. This is asituation which exists when the source of transmission lies on theperpendicular bisector of the line connecting the two active antennas.For this in phase mode of reception, the relative phases of the signalsappearing at the collector of transistor Q19 and its counterpart inchannel B are determined by the settings of variable resistor R75 andits counterpart in channel B. These variable resistors are set so thatthe signals appearing at the collectors of transistor Q19 and itscounterpart in channel B are 180° out of phase when the two receivedsignals are in phase. Under these circumstances the signal appearing atthe output of NAND gate U7 and represented by waveform 215 is similar tothe signal appearing at the output of NAND gate U3 but with its briefnegative going excursions displaced in time from those of waveform 214.Under these conditions the outputs of the latch circuit comprised ofNAND gates U4 and U8 are represented by waveforms 216 and 217, waveform216 representing the output of NAND gate U4. For this condition, whenthe signals received by the two antennas are in phase, the outputs ofthis latch circuit are each symmetrical square waves and are 180° out ofphase with each other.

Waveforms 216 and 217 are each inverted by transistors Q21 and Q22respectively. The output of transistor Q21 is low pass filtered by thecombination of resistor R94 and capacitor C108 while the output oftransistor Q22 is low pass filtered by the combination of resistor R96and capacitor C109. Each of these low pass filters has an extremely longtime constant so that the output labeled "phase (+) signal" iseffectively a positive DC voltage, the level of which is proportional tothe relative off times of the signal appearing at the output of NANDgate U4 and represented by waveform 216. For the situation depicted byFIG. 13, that is, when the two received signals are in phase, the DClevels appearing at the output terminals labeled phase (+) signal andphase (-) signal are equal since both waveforms 216 and 217 have equalon and off times. These two terminals are connected to oppositeterminals of direction meter 142 of FIG. 7 which in the preferredembodiment is a 500-0-500 microampere ammeter. Under these circumstancesdirection meter 142 reads mid scale to indicate that the transmitter iseither directly in line with the receiver or directly abreast of thereceiver in accordance with whether the left/right or front/rearantennas are active. If, however, the signals received by the activeantennas are not in phase, then the outputs of NAND gate U4 and U8 willbecome asymmetrical and the DC voltage at one of the phase detectoroutputs will increase while the DC voltage at the other phase detectoroutput will decrease. The resultant non-zero voltage appearing acrossthe terminals of direction meter 142 will cause the panel meter toindicate either upscale or downscale in accordance with the relativephases of the received signals.

Portions of the display circuits which form part of the beacon trackingsystem are illustrated in FIG. 14. Since many of the elements of thesecircuits are well known in the art and in the interest of clarity ofexposition, these circuits have been illustrated largely in blockdiagram form. The output signal from second IF amplifier 124, appearingon line 130 of FIG. 7, is coupled to amplifier and detector 230. Thisunit comprises a single stage amplifier coupled into a diode detector.The time constants of this diode detector are selected to permit it tofollow a 570 hertz amplitude modulation envelope on the 455 khz carrier.The detected signal drives limiter 232 so as to provide an output signalwhich alternates between the upper and lower limits of the limiter.Limiter 232 as well as the various comparators appearing in FIG. 14 mayeach be similar to a model LM311N integrated circuit manufactured byNational Semiconductor Corporation of Santa Clara, Cal. The output oflimiter 232 is filtered by 570 hertz bandpass filter 234 which in thepreferred embodiment comprises an operational amplifier with a 570 hertznotch filter in the feedback loop. The filtered signal is coupled todiode detector 236 which produces a positive DC voltage when a 570 hertzsignal is present. This DC voltage, which is proportional to theamplitude of the 570 hertz signal, is matched in comparator 238 againsta selectable reference voltage. When the DC voltage corresponding to the570 hertz signal exceeds the selectable threshold voltage, thecomparator produces a positive output voltage which gates audio signalsthrough NAND gate 240 to audio driver 242. Audio driver 242 producesexcitation at the required level to speaker 152. The display circuitsdescribed to this point permit excitation of speaker 152 only when thereceived signal is modulated at a 570 hertz rate and when the modulationlevel exceeds a selectable threshold. In this way the system rejects allsignals other than those produced by the transmitter which forms a partof the beacon tracking system.

A second input to the display circuits of FIG. 14 is the AGC signalproduced by second IF amplifier 124 and appearing on line 154 of FIG. 7.This AGC signal is matched against a selectable reference voltage indifferential amplifier 244. The output of this amplifier, which is a DCvoltage proportional to the difference between the AGC signal level andthe selectable reference voltage, is used to control the frequency ofvoltage controlled oscillator 246. Voltage controlled oscillator 246 isa voltage controlled astable multivibrator of the type well known in theart. Its square wave output provides a second input to NAND gate 240 andhas a frequency which varies within the audio range in accordance withthe level of the AGC signal. As the AGC signal level increases,reflecting a stronger signal from a near transmitter, the frequencyproduced by the voltage controlled oscillator 246 increasesproportionally. The specific range of frequencies to be covered isdetermined by the selectable reference voltage which forms one input todifferential amplifier 244. Assuming for the moment that the logic levelappearing on line 248 is high, then when the logic signal produced bycomparator 238 is also high, the output of voltage control oscillator246 is gated through NAND gate 240 into audio driver 242 to produce anaudible tone in speaker 152. The frequency of this tone is directlyindicative of the proximity of the transmitter.

The phase (+) signal produced by the phase detector circuit of FIG. 12forms one of the inputs to each of comparators 250 and 252. This phase(+) signal will have some nominal DC level when the signals received bythe two active antennas are in phase. Comparator 250 is also responsiveto a selectable reference voltage appearing on line 254. This voltage ischosen to be some DC value less than the aforementioned nominal DClevel. Similarly, comparator 252 is also responsive to a selectablereference voltage appearing on line 256. This selectable referencevoltage is chosen to be some DC value above the aforementioned nominalDC level. When the signals received by the two active antennas are inphase or when their phase difference is less than some predeterminedlevel, the amplitude of the phase (+) signal will be intermediate to thevoltage level appearing on lines 254 and 256. Under these circumstanceswith the comparator input polarities connected as shown in FIG. 14, theoutput signals provided by both of these comparators will be in the highstate. This results in a low level logic signal at the output of NANDgate 258 which in turn insures that the logic signal appearing on line248 will remain in the high state and permit the output signal ofvoltage controlled oscillator 246 to be coupled through NAND gate 240.If, however, the amplitude of the phase (+) signal falls below theamplitude of the signal on line 254 or becomes larger than the amplitudeof the signal appearing on line 256 (as it will when the phasedifference between the two received signals becomes large) then theoutput signal generated by one of comparators 250 and 252 will switch tothe low state. This results in a high level logic signal appearing online 260, which in turn gates the signal generated by astablemultivibrator 262 through NAND gate 264. Astable multivibrator 262provides an output square wave having a period in the range one-quarterto one-half second. This low frequency square wave appearing on line 248acts as a periodic gating function to NAND gate 240. The end result isthat the audible tone produced by speaker 152 becomes intermittent towarn an operator that the vector to the transmitter has departed fromthe receiver axis by more than a preselected threshold.

The phase (+) signal is also compared with the phase (-) signal incomparator 266. The output of comparator 266 provides one input to NANDgate 268 and, after inversion by inverter 270, provides an input to NANDgate 272. NAND gates 268 and 272 are enabled when the signal appearingon line 274 is in the high state indicating a transmitter signal hasexceeded a threshold of comparator 238. The third input to NAND gates268 and 272 is a voltage reference level established by resistor R99,capacitor C112 and zener diode VR4 when these components areelectrically connected to a DC supply voltage by the closure of switch30. Switch 30 is the front/rear switch located on the front panel of thedisplay unit as shown in FIG. 2. Depending on the relative amplitude ofthe phase (+) signal and phase (-) signal the output of one of NANDgates 268 and 272 will be in the high state while the output of theother will be in the low state. If, for example, the output of NAND gate268 is in the low state then the output of inverter 274 will be in thehigh state and will enable light driver 278 to energize "front" lamp 32to indicate to an operator that the transmitter being detected islocated in front of the receiver. When the opposite situation exists"rear" lamp 34 will be illuminated.

There has been disclosed a beacon tracking receiver which providessignals representative of the direction to and the distance to a remotetransmitter. The receiver includes two intermediate frequency channelswhich are switchably responsive to a left/right or fore/aft pair ofantennas. The IF channels feed a phase comparator which provides asignal representative of the direction of the transmitter. An AGC signalin one of the channels is employed as a measure of the proximity of thetransmitter. While the preferred embodiment of the invention has beendisclosed there may be suggested to those skilled in the art minormodifications which do not depart from the spirit and scope of theinvention as set forth in the appended claims.

What is claimed is:
 1. A receiver for receiving R.F. signals from asubminiature transmitter and producing signals indicative of thedistance and position of the transmitter relative to the receivercomprising:a. first, second and third omnidirectional antennaepositioned preselectively on a movable support to detect transmittedR.F. signals; b. first, second and third local oscillators for producingR.F. signals at preselected frequencies; c. first and second tripleconversion channels, each channel including a preamplifier, a firstmixer coupled to the outputs of the preamplifier and first localoscillator for producing IF signals at a first preselected frequency, afirst IF amplifier for amplifying the IF signals, a second mixer coupledto the outputs of the first IF amplifier and second local oscillator forproducing second IF signals at a preselected frequency, a second IFamplifier for amplifying the second IF signals, an automatic gaincontrol circuit coupled to the second IF amplifier and first IFamplifier for feeding back the output of the second IF amplifier to thefirst IF amplifier for increasing the response of the first amplifier toa wide dynamic range of input signals, a third mixer coupled to theoutputs of the second IF amplifier and third local oscillator forproducing third IF signals at a preselected frequency and a third IFamplifier for amplifying the third IF signals, said first tripleconversion channel having its preamplifier input coupled to the firstomnidirectional antenna and its automatic gain control circuit providinga distance measuring signal for a means for determining distance to theR.F. transmitter responsive to the strength of the detected R.F.signals; d. a relay means for selectively coupling the second and thirdomnidirectional antennae to the preamplifier of the second tripleconversion channel; and e. a phase detector coupled to the outputs ofthe third IF amplifiers of the first and second triple conversionchannels for comparing the phases of the R.F. signals from the first andsecond omnidirectional antennae detected signals to produce signalsindicative of the left/right position of the R.F. transmitter relativeto the receiver, and for comparing the first and third omnidirectionalantennae detected signals to produce signals indicative of thefront/rear position of the R.F. transmitter relative to the receiver. 2.The receiver of claim 1 having three antennas located at the vertices ofa right triangle.
 3. The receiver of claim 1 wherein the first andsecond antennas are separated by approximately one-half wavelength atthe frequency of the signal detected by said antennas and the thirdantenna is separated from at least one of said first and second antennasby approximately one-half wavelength.
 4. A receiver according to claim 1wherein the relay means for selectively coupling the second and thirdomnidirectional antennae to the preamplifier of the second tripleconversion channel comprises a coaxial relay and a switch operablyconnected to the coaxial relay, said coaxial relay responsive toactivation of the switch for selectively coupling the second and thirdomnidirectional antennae to the preamplifier of the second tripleconversion channel.